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Lesson plan 38: Geographic architecture
Allocate a geographic latency budget across ownership, execution, replication, and client delivery.
Placement: Part 7: Building and operating applications on these engines.
Prerequisites: Lesson 37: Application data architecture, plus the earlier dependency chain.
Learning outcomes
- Allocate a geographic latency budget across ownership, execution, replication, and client delivery.
- Compare regional ownership, global reads, and global writes under the required consistency contract.
- Explain how failover placement changes both latency and failure independence.
- Separate a general mechanism, a documented implementation choice, and an illustrative numerical assumption.
Teaching sequence
- Establish the motivating situation: A global inventory invariant is placed on a leader far from most buyers.
- Introduce the vocabulary and the correctness obligation before an optimization. Define placement, regional ownership, global reads.
- Develop the mechanism: Draw local and cross-region paths; choose ownership and read freshness based on the contract.
- Follow the complete worked example: Two sequential 80 ms WAN round trips contribute 160 ms before processing. Explain the assumptions, intermediate units, and limiting interpretation.
- Extend the scenario to changed load, skew, or failure, explaining global writes, locality, distance, failover.
- Connect the mechanism to a documented implementation and an alternative with a different workload fit. Close by stating the remaining cost and the next lesson dependency.
Required chapter material
Use a complete execution trace or worked calculation, supported by an annotated static diagram or table. Include language-neutral pseudocode or labeled SQL when it clarifies the mechanism. State preconditions, ownership, visibility, and failure boundaries for algorithms. Describe operational costs as well as the normal path. All questions raised in the narrative must be resolved in the text.
Visual explanation
A region-placement diagram labels client distance, leader location, quorum paths, and a failover target rather than presenting geography as a feature toggle. The accompanying prose explains the arrows, state changes, or table entries so the lesson remains understandable in a plain Markdown reader.
Editorial checks
Confirm that the numerical example agrees with its units and assumptions. Check that a faster path preserves the stated contract, or explicitly identify the changed guarantee. Keep hypothetical figures separate from product measurements. Explain both an appropriate use and a limitation of the case design. Check terminology against the glossary and links against the course index.